In recent years, positioning systems have been put into practical use, such as the GPS, or are planned to be deployed, such as the prospective satellite-based Galileo, and are expected to gain more and more popularity.
The GPS is a well known multiple-satellite based radio positioning system in which each GPS satellite transmits data that allows a GPS receiver to precisely measure the distance from selected ones of the GPS satellites to its antenna and to thereafter compute position, velocity, and/or time parameters to a high degree of accuracy, using known triangulation techniques. The satellites are positioned in a constellation such that typically seven, but a minimum of four, satellites are observable by a receiver anywhere on or near the earth's surface. Each satellite's signals are synchronized to an atomic clock and the normal user apparatus' clock is in error by a clock bias. By measuring the apparent satellite signal propagation times from four satellites rather than three, the redundancy can be used to solve for clock bias and the position calculated. Prior to correction for the user's clock bias, the apparent ranges of the satellites are all in error by a fixed amount and are called pseudoranges.
Each satellite transmits signals on a center frequency of 1575.42 MHz within a band known as L1, using spread spectrum techniques that employ a coarse/acquisition (C/A) pseudo random noise (PRN) code. The codes have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 msec. The code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique Gold code. The signals received at a user's receiver have a bandwidth of approximately 2 MHz and a typical signal to noise ratio of about −20 dB.
Superimposed on the C/A code is the navigation message, i.e. a direct sequence spread spectrum, 50 bit/s binary phase shift keyed data signal. It contains, among others, the GPS system time, the satellite clock correction parameters, ephemeris data for the particular satellites being tracked and almanac data for all of the satellites in the constellation. The ephemeris consists of detailed information about the satellite's own course over the next two hours, the almanac consists of less detailed information about the complete satellite constellation for a longer period and the clock correction parameters allow the user apparatus to correct for the errors of the GPS satellite's own clock.
There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites, and (2) computation of the position (and other navigation data) of the receiver using these pseudoranges and satellite timing and ephemeris data. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked.
Virtually all known GPS receivers utilize correlation methods to compute code-phases (see below). For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or lowpass filters, the product in order to obtain an indication of the presence of the signal. This process is termed a “correlation” operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal code and the local code replica (‘code-phase’). The initial determination of the presence of such an output is termed “acquisition.” Once acquisition occurs, the process enters the “tracking” phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output and to refine the code-phase and carrier frequency measures. The correlation output during the tracking phase may be viewed as the GPS signal with the pseudorandom code removed, or, in common terminology, “despread”, which represents the navigation signal.
In addition, since the satellites are each moving at a speed in excess of 3 km/s, the GPS signals are received with a Doppler frequency offset from the GPS centre frequency. As a result, a mobile GPS receiver has to be capable of receiving signals with frequencies having an offset greater than ±4 KHz from the GPS centre frequency. To recover the data and measure the propagation time of the satellite signals, the GPS receiver must cancel or allow for the Doppler frequency offset and generate the C/A code relevant to each satellite.
Typically, in conventional standalone GPS receivers all navigation processing activities (e.g. acquisition, tracking and navigation solution) occur at the receiver, which outputs the position and velocity of the receiver. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, especially if received signals are weak, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications. To improve acquisition time, most GPS receivers utilize a large number of correlators which allows a parallel search for correlation peaks.
Another limitation of conventional GPS receivers is that their operation, in order to achieve low error rates, is limited to situations in which multiple satellites are clearly in view, without obstructions or attenuations, and where a good quality antenna is properly positioned to receive such signals. A standalone conventional GPS receiver requires strong signals (>=−135 dBm) over long time periods to perform its functions without any external assistance. As such, they normally are unusable in portable, body mounted applications; in areas where there is significant foliage or building blockage (e.g., urban canyons); and in indoor applications.
Cellular phones for mobile communication through a mobile network which comprise, or are associated to, GPS receivers are rapidly gaining popularity. The association of the GPS receivers with cellular phones, together with the diffusion of location based services, which may typically need a fix of the position of a user apparatus, are driving the need of having a fix of the position of the user apparatus based on a signal of a positioning system in urban and/or indoor environments, which are typical environments for the use of cellular phones and of mobile network services.
The Applicant has noted that prior art GPS receivers are not suitable for fixing the position of a user apparatus in cellular-related environments, such as in particular in urban or indoor environments.
The urban and indoor environments are typically characterized by difficult receiving conditions. For example, the indoor environment typically shows receiving conditions characterized by strong attenuation of the received positioning signal, which may suffer an additional attenuation of about 10-20 dB with respect to strong field receiving conditions (e.g., open field), wherein typically the minimum signal level is at about −135 dBm. On the other hand, the urban environment (in particular, e.g., ‘urban canyon’ environment) typically shows the problem of restricted visibility of some of the satellites (‘sky obstruction’) and/or the interference problem, including the multipath problem wherein the positioning signal is reflected by a plurality of surfaces before reaching the receiver. The interference terms lead to an increase of the attenuation and/or noise of the positioning signal and/or an increase in the positioning error. Environments showing difficult receiving conditions are referred to as ‘weak signal’ environment; while, in the opposite case, the expression ‘strong signal’ environment is used.
On the other hand, in cellular-related environments the user expects a high quality of service, which means, among others, a high degree of availability of the positioning service and/or a short time-to-first-fix (TTFF), i.e. the time elapsed before having the first fix starting from the request of the fix by the user. The TTFF is particularly important in case the fix of the position based on a positioning signal is done in connection with location based services. In fact, location based services are designed to provide very good user-experience, which includes a very short time to wait before the service is provided.
The requirement of a short TTFF is more demanding in the case (typical for mobile positioning receiver due to the strict power consumption requirements) of the so called ‘cold start’, i.e. in case the positioning receiver is switched on only upon a request for a fix. In fact, in this case the tracking of the positioning signal takes longer time than that in the case of ‘hot start’ since no prior information is available in the receiver.
SnapTrack, Inc. has developed a wireless assisted GPS technology for location determination which is air interface independent. For example, U.S. Pat. No. 6,542,821 describes a GPS receiver having circuitry configured to receive and process the pseudorandom sequences during sky blockage conditions. The circuitry processes the pseudorandom sequences by digitizing and storing a predetermined record length of the received sequences and then performs fast convolution operations on the stored data to determine the pseudoranges by way of a programmable digital signal processor.
In the above patent, it is also described a method for determining the position of a remote GPS receiver by storing GPS satellite information, including Doppler, in the remote unit. During blockage conditions, the remote unit uses this information and sampled GPS signals from in view satellites to subsequently compute pseudoranges to the satellites using fast convolution operations. The computed pseudoranges may then be used to determine the position of the remote unit. The position determination can occur at the remote unit or at a basestation. Where the position determination is performed at a basestation, the remote unit transmits via a data link the pseudoranges to the basestation, which can combine this information with the satellite ephemeris data to complete the position calculation. Where the position determination is performed at the GPS receiver, either the Satellite Data Messages are retransmitted to the GPS receiver from the basestation so that the GPS receiver may combine this information with the pseudorange measurements to compute its position or the GPS receiver itself may gather the satellite ephemeris data, which typically is valid for one to two hours, from the reception of GPS signals in the normal manner that is commonly practiced in the art using a conventional GPS receiver.
In the cited U.S. Pat. No. 6,542,821, in order to achieve high sensitivity, a very long portion of waveform, typically 100 ms to 1 s, is processed.
The Applicant has found that computing the pseudoranges at the GPS receiver has the drawback of having a limited computational capacity for the pseudorange calculation itself, especially in view of the requirement to minimize the power consumption. According to the Applicant, this may in turn limit the fix availability, at a desired fixing accuracy, and the reduction of the TTFF. In addition, the above solution results in a complex hardware and/or software of the GPS receiver, thus increasing its cost and/or reliability.
In the art alternatives to the conventional standalone GPS receiver are known which consist in the GPS translators or transdigitizers, wherein the GPS signal is remoted by translation or variations thereof and the signal is tracked at a ground processing facility where the object position and velocity are derived. These translators or transdigitizers typically include only the antenna assembly and RF assembly portions of a GPS receiver and are typically employed in applications where expendable sensors are required. Known transdigitizers retransmit the digitally sampled GPS signal at 2 Msps using quadraphase modulation. In accordance with this approach, significant bandwidth is always required to transmit the translated signal.
U.S. Pat. No. 5,379,224 describes a tracking system using GPS satellites, comprising a GPS sensor module that supplies the data required to locate a particular object, a one-way telemetry link, and a data processing workstation to process the data and display the object position and velocity. The GPS sensor module comprises an antenna and a sensor. The sensor digitally samples the signals from visible GPS satellites and stores this data in a digital buffer. No processing functions are performed by the sensor, thereby permitting significant reductions in the cost thereof. The raw satellite data stored in the buffer are transmitted back to the data processing workstation. Using this set of raw satellite data, the position and velocity of the sensor can be determined at the time the data was recorded. If a 20 kHz data link is used and the GPS signals are sampled at 2 Mbps, a 1-second set of GPS data can be provided every 100 seconds, or a 0.5-second set of GPS data every 50 seconds, or a 0.1-second set of data every 10 seconds.
The Applicant has found that it is desirable to provide a user with a mobile network positioning service based on a positioning signal, which is highly available in both strong and weak signal receiving conditions, while minimizing the bandwidth requirement of the mobile network service. In fact, one important factor for the choice of a service is the price and, whichever is the mobile network connection between the network equipment and the mobile user apparatus, in general the higher is the traffic, the higher is the charge for the service. In particular, for (e.g. packet) data mobile network connection, whichever is the bandwidth, i.e., the data transfer rate provided by the connection itself, the greater is the amount of data to be transferred, the higher is in general the price.
The Applicant has thus faced the problem of providing a method, a system and a service based on the method, for fixing the position at a network equipment of a mobile user apparatus on the basis of a signal of a positioning system received at the user apparatus, wherein the fixing is provided with good availability, for a given fixing accuracy, also in weak signal receiving conditions and a data transfer traffic, associated to the fixing, through a network connection between the user apparatus and the network equipment is optimized at a suitably low level. Moreover, the fixing should preferably be characterized by a short time-to-first-fix, also in cold start conditions. In addition, the fixing of the position should be desirably low-cost, reliable and fast.